Unauthorized copying of recorded works has been a problem to the rightful owners of those works for centuries, providing a reason for the promulgation of copyright laws. With advances in technology, however, the ability to copy has become increasingly feasible and affordable. In particular, modern storage of information is often digital in nature, which allows repeated copying without incremental deterioration each time a copy is made. In addition, prerecorded audio, video or software programs often sell millions of legitimate copies, providing an incentive to those who may be tempted to risk running afoul of copyright laws to produce and sell pirated copies.
Several systems have been developed to thwart unauthorized copying. In U.S. Pat. No. 5,182,770, Medveczky et al. teach an integrated software piracy prevention system incorporating identification codes for installation and software components. The licensed software has an embedded program identification code, and an attached hardware device also has an embedded code, the hardware code and software code used to encrypt and decrypt the program and combining to provide an identification signature corresponding to a specific password. However, provision is not made for the situation in which the encryption code is broken and a number of copies of the software including the code are produced. U.S. Pat. No. 5,138,712 to Corbin provides a different approach to licensing software, in which a license server acts as an intermediary between an application program and a licensing library. This approach works best for computer networks allowing a restricted number of users to access the software at any given time.
In U.S. Pat. No. 5,291,554, Morales describes a pay per view television system that includes cryptographic keys for restricting access to movies and other video programs broadcast from a satellite to individual subscribers. Copying is possible once the programs have been decrypted by a subscriber. For communication systems such as radios and cellular telephones which are frequently operated, rekeying encryption codes of individual units such as telephones is employed to prevent unauthorized use of the communication system, as described in U.S. Pat. No. 5,173,938 to Steinbrenner et al. Further, U.S. Pat. Nos. 5,146,497 and 5,150,408 to Bright teach methods for providing such rekeying which corrects for delays in rekeying caused by communication, including resending rekeying messages when acknowledgement of the rekeying is not received from all of the communication units. U.S. Pat. No. 5,301,232 to Mulford teaches a key management system that determines whether individual communication units are using an old encryption scheme.
Despite these improvements, a need still exists to provide a better method for preventing unauthorized copying and distribution of prerecorded material such as audio, video and software programs. A particular object of the present invention is to provide protection against unauthorized copying and distribution, even in the event an encryption code designed to protect against such copying is broken.